Optical Fiber Cable Having A Deformable Coupling Element

ABSTRACT

Disclosed is an optical fiber cable that includes optical fibers and a deformable coupling element enclosed within a buffer tube. The coupling element is formed from a deformable yet substantially incompressible material that is capable of releasably and intermittently coupling the optical fibers to the buffer tube in various orientations. The design of the coupling element layer permits coupling of the optical fibers to the buffer tube without the use of a compressive cushioning layer and yet permits localized movement the optical fibers relative to the buffer tube to account for disparate thermal expansion and to accommodate optical fiber placement.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application is a division of commonly assigned U.S. patentapplication Ser. No. 12/643,222 for an Optical Fiber Cable Having aDeformable Coupling Element, filed Dec. 21, 2009 (and published Apr. 22,2010, as Publication No. 2010/0098388 A1), now U.S. Pat. No. 8,036,509,which itself is a division of commonly assigned U.S. patent applicationSer. No. 12/146,526 for an Optical Fiber Cable Having a DeformableCoupling Element, filed Jun. 26, 2008 (and published Jan. 1, 2009, asPublication No. 2009/0003781 A1), now U.S. Pat. No. 7,639,915, whichitself claims the benefit of U.S. Provisional Patent Application Ser.No. 60/946,744, for an Optical Fiber Cable Having a Deformable CouplingElement (filed Jun. 28, 2007). Each of the foregoing patent applicationsand patent application publications is hereby incorporated by referencein its entirety.

FIELD OF THE INVENTION

The invention relates to an optical fiber cable that contains opticalfibers inside a buffer tube.

BACKGROUND

Optical fiber cables are used to transmit information includingtelephone signals, television signals, data signals, and Internetcommunication. To preserve the integrity of the signal transported byoptical fiber cables, certain design factors warrant consideration.

First, forces may develop on the optical fibers due to contact withrough, hard, or uneven surfaces within the optical fiber cables. Suchcontact, for example, may result from thermal cable expansion orcontraction, which can cause microbending and macrobending effects.This, in turn, can lead to signal attenuation or signal loss. Layers ofprotective coatings and claddings around the optical fibers can help toreduce the forces that cause these unwanted effects.

Second, the optical fibers are typically coupled to the surroundingbuffer tube in some way. This coupling prevents the optical fibers frompulling back inside the buffer tube as a result of processing,installation, handling, or thermally induced dimensional changes. Notonly can these effects hamper accessibility to the fibers duringconnection operations (e.g., splicing), but also insufficient couplingcan lead to excess and/or unevenly distributed optical fiber length(e.g., optical fibers accumulating in a confined space). Suchaccumulation may cause bending or otherwise force contact between theoptical fibers and other cable elements, which can likewise lead tomicrobending and macrobending.

Third, optical fiber cables are typically used with electronic devices.If water intruding into the cables can spread (e.g., flow) along thelength of the cables to these electronic devices, severe damage to theelectronic systems may result. It is also thought that the formation ofice within an optical fiber cable can impose onto the optical fiberslocalized microbending-inducing forces or macrobending-inducing forces.Fillers and water-blocking layers within the cables can impede themovement of water within the cables and thereby limit such damage.

The undesirable effects of signal loss, coupling failure, and waterdamage can be reduced through the use of protective layers and couplingelements. The addition of these layers, however, can lead to largercables, which are not only more costly to produce and store but alsoheavier, stiffer, and thus more difficult to install.

Manufacturers have typically addressed these problems by employingwater-blocking, thixotropic compositions (e.g., grease or grease-likegels). For example, filling the free space inside a buffer tube withwater-blocking, petroleum-based filling grease helps to block theingress of water. Further, the thixotropic filling grease mechanically(i.e., viscously) couples the optical fibers to the buffer tube.

That usefulness notwithstanding, such thixotropic filling greases arerelatively heavy and messy, thereby hindering connection and splicingoperations. Consequently, filling greases carry certain disadvantages.

Various designs for dry cables have been developed to eliminate fillinggreases while providing water-blocking and coupling functions. Forexample, in a totally dry cable, filling grease may be replaced by awater-swellable element (e.g., tape or yarn carrying a water-swellablematerial).

Unfortunately, in practice, the water-swellable elements used in thesedesigns may not provide for sufficient coupling of the optical fibers tothe buffer tube. That is, the optical fibers are free to pull backinside the cable when the cable is installed or exposed to temperatureextremes.

Purported solutions to this problem have been proposed, typicallyinvolving the inclusion of a cushioning material, such as polymeric foam(e.g., polyurethane foam), that either surrounds the optical fibers oris layered on the water-swellable tape. To achieve the desiredmechanical coupling, though, the cushioning is sized such that it iscompressed between the optical fibers and the buffer tube. In this way,the cushioning promotes frictional coupling of the optical fibers to thebuffer tube.

Although frictional coupling is effective in preventing relativemovement between the optical fibers and the buffer tube, the opticalfibers may experience microbending or macrobending when the buffer tubecontracts due to cooling. This may result in optical signal attenuationor signal loss. Further, the coupling pressure exerted on the opticalfibers by the foam cushioning may diminish over time due to therelaxation or degradation of the polymeric foam.

Accordingly, there is a need for a dry optical fiber cable in whichoptical fibers are coupled to a buffer tube in a way that does not exertundue stresses on the optical fibers and is reliable over the life spanof the cable.

SUMMARY OF THE INVENTION

In one aspect, the invention embraces a cable structure that providescoupling between buffer tubes and optical fiber elements. In thisaspect, the present invention includes a coupling element that is bondedto one of the material layers in the cable, such as the inner wall of abuffer tube or a water-swellable tape layer.

This coupling element is made of a sufficiently elastic material so asto deform under the load of the optical fiber element (e.g., havingelongation to break of at least about 100 percent at standardtemperature and pressure) and has sufficient strength so as not to tearunder the weight of the optical fiber element (e.g., having tensilestrength of at least about 100 psi at standard temperature andpressure). It is typically dry. Moreover, the coupling element is sizedso as to not fill the entire space between the buffer tube and theoptical fiber element. Consequently, the coupling element typicallycontacts the optical fiber element at discrete locations along thelength of the optical fiber element.

For example, under normal circumstances (e.g., horizontal cableplacement), the optical fiber element contacts the deformable profile ofthe coupling element at distinct points along the length of the opticalfiber element. (Stated differently, the optical fiber element willtypically not rest upon the coupling element in a fully extended form).The contact points between the optical fiber element and the couplingelement are not fixed and will typically change if there is movement ofa cable component (e.g., thermal contraction of the buffer tube.) Theoptical fiber element's intermittent contact (i.e., discontinuouscontact) with the coupling element is caused in part by the tendency ofthe optical fiber element to assume a non-linear (e.g., curved)orientation within the buffer tube.

Free space between the optical fiber element and the other components(e.g., layers) within the buffer tube allows the optical fiber elementto move more or less freely within the cable. For example, although theglass of the optical element and the plastic of the buffer tube mayrespond differently to thermal loads, the optical fiber element is notfixedly secured to the coupling element or buffer tube. Consequently,the optical fiber element is not forced to move as the buffer tubethermally expands or contracts.

This free space can also cause problems, however, if the optical fiberelement is not adequately coupled to the buffer tube. For example, undervertical placement, an optical fiber element having at least some excessoptical fiber length can assume a generally non-linear orientationwithin the cable (e.g., helical, sinusoidal, or often non-uniformorientation) and will tend to bunch, slump, gather, or otherwise collectat lower portions of the cable (i.e., because of gravitational pull).This bunching, slumping, gathering, and collecting (i.e., excess length)can lead to bending and other pressures that can cause signalattenuation.

The design of the coupling element according to the present inventionhelps to reduce such undesirable collection. At discrete (but non-fixed)locations along the length of the optical fiber cable, the optical fiberelement will contact the coupling element (in part due to the non-linearform that the optical fiber element assumes because of excess length).Under such conditions, the coupling element will deform under theapplied force of the optical fiber element, thereby creating ashelf-like protrusion (e.g., a bulge) that helps to support the opticalfiber element. This deformation can be modest or substantial depending,of course, upon the particular characteristics of the coupling elementand the applied force.

It is thought that the support provided by this shelf-like deformationhas an axial normal force component in addition to any radial normalforce and frictional force. In contrast to the frictional couplingprovided by compressed cushioning layers, the optical fiber elementcontacts the other elements in the buffer tube only at discretelocations rather than along the entire length of the optical fiberelement. As a result, the design of the coupling element according tothe present invention limits the occurrence of the kinds of unwantedpressures that can cause microbending or macrobending in the opticalfiber element.

The design of the coupling element, therefore, serves to couple theoptical fiber element to the buffer tube without the use of acompressive, frictional cushioning layer. In addition, the optical fibercable according to the present invention allows the optical fiberelement to move relative to the buffer tube on a small, localized scale.This, in turn, helps to minimize the development of pressures on theoptical fibers that can occur from cable placement or from disparatethermal expansion of the cable components.

The foregoing, as well as other characteristics and advantages of theinvention and the manner in which the same are accomplished, is furtherspecified within the following detailed description and its accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross-sectional view of an optical fiber cableaccording to one embodiment of the present invention.

FIG. 2 depicts a cross-sectional view of an optical fiber cableaccording to one embodiment of the present invention to illustrate theoptical fiber cable's coupling space.

FIG. 3 depicts a cross-sectional view of an optical fiber cableaccording to one embodiment of the present invention to illustrate theannular free space between the optical fiber element and its surroundingelements.

FIG. 4 depicts a partial sectional view of an inner wall of the opticalfiber cable according to the present invention along line 2-2 of FIG. 1.

FIG. 5 depicts a perspective view of an optical fiber cable according toone embodiment of the present invention in which the coupling element isintermittently spaced along the length of the optical fiber cable. (Forclarity, this figure omits the respective optical fiber element.)

FIG. 6 depicts a cross-sectional view of an optical fiber cableaccording to another embodiment of the present invention.

FIG. 7 depicts a cross-sectional view of an optical fiber cableaccording to yet another embodiment of the present invention.

FIG. 8 depicts a cross-sectional view of an optical fiber cableaccording to yet another embodiment of the present invention.

FIG. 9 depicts a cross-sectional view of an optical fiber cableaccording to yet another embodiment of the present invention.

FIG. 10 depicts the nominal coupling diameter of a coupling element andthe maximum cross-sectional width of an optical fiber element for anexemplary, elliptical optical fiber cable.

FIG. 11 depicts an exemplary optical fiber cable in which the innerdiameter of the buffer tube is greater than the combined thicknesses ofall elements positioned within the buffer tube (i.e., thewater-swellable element, the coupling element, and the optical fiberelement).

DETAILED DESCRIPTION

The present invention is described herein with reference to theaccompanying drawings. As will be appreciated by those having ordinaryskill in the art, these drawings are schematic representations, whichare not necessarily drawn to scale. This invention may be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. The embodiments disclosed are provided toconvey the scope of the invention to those having skill in the relevantart.

In one aspect, the invention embraces an optical fiber cable thatprovides satisfactory coupling of optical fibers and a surroundingbuffer tube but in a way that reduces unwanted microbending andmacrobending effects. In another aspect, the invention embraces methodsof making such optical fiber cables.

FIG. 1 depicts a cross-sectional view of one embodiment of an opticalfiber cable 10 according to the present invention. In this exemplaryembodiment, an optical fiber element 12 is disposed within a buffer tube16. The optical fiber element 12 itself includes at least one opticalfiber, typically a plurality of optical fibers (e.g., a ribbon stack).The optical fiber element 12 is typically manufactured with rotationaltwist.

In this exemplary embodiment, a water-swellable element 14 (e.g., awater-blocking material) is positioned adjacent to the inner wall of thebuffer tube 16 between the optical fiber element 12 and the inner wallof the buffer tube 16. The water-swellable element 14 helps to block theingress of water into the core of the optical fiber cable 10 or, ifwater intrusion occurs, helps to prevent the movement of water along thelength of the optical fiber cable 10. The water-swellable element 14 maybe secured to the buffer tube 16, for example, by an adhesive, bymelt-bonding part of the water-swellable element 14 to the buffer tube16 during extrusion, or by frictional coupling of the water-swellableelement 14 and the buffer tube 16.

Further referencing the exemplary embodiment of FIG. 1, a couplingelement 18 is positioned adjacent to the optical fiber element 12 (i.e.,between the optical fiber element 12 and the buffer tube 16). Asdepicted in FIG. 1, the coupling element 18 is configured (e.g., sized)to provide sufficient coupling space to accommodate the optical fiberelement 12 without forced contact. In other words, the optical fiberelement 12 can fit within the coupling element 18 such that there isfree space between the optical fiber element 12 and the coupling element18 around the full perimeter of the optical fiber element 12 (i.e.,“annular free space”).

As used herein, the concept of “coupling space” refers to the innercable region (i.e., the free space) in which the optical fiber element12 is positioned. FIG. 2 illustrates the “coupling space” of the opticalfiber cable 10 depicted in FIG. 1. (For clarity, FIG. 2 omits theoptical fiber element 12.) As will be recognized by those havingordinary skill in the art, the coupling space is typically defined bythe configuration of the coupling element 18.

Moreover, as used herein in this context, the term “annular free space”is intended to characterize unfilled space that can exist between theoptical fiber element 12 and its surrounding elements (e.g., thecoupling element 18) around the entire perimeter of the optical fiberelement 12, regardless of the respective shapes of the optical fibercable 10 and its components (e.g., a rectangular ribbon stack within around coupling insert as depicted in FIG. 1). In this regard, the term“annular free space” as used herein is not limited to the regular gapbetween two concentric tubes (or casings) having circular cross-sections(i.e., a perfect annulus).

Likewise, FIG. 3 demonstrates how the optical fiber cable 10 of FIG. 1can define “annular free space” surrounding the optical fiber element 12(and thereby satisfy the concept of “annular free space” as usedherein). In general, an optical fiber element 12 that is sized andconfigured to fit within a buffer tube 16 in a way that does not requirecontact with its surrounding elements—typically the coupling element18—will be capable of defining annular free space within the opticalfiber cable 10 (i.e., contact-free placement).

Those having ordinary skill in the art will understand that theformation of annular free space will occur, if at all, over discretesegments of the optical fiber cable 10. Over any extensive length of theoptical fiber cable 10, some contact between the optical fiber element12 and the coupling element 18 is virtually assured (i.e., opticalcables as used in the field are subjected to bending). (Indeed, asdiscussed herein, it is interference contact that promotes advantageouscoupling of the optical fiber element 12 and buffer tube 16.) To theextent the optical fiber element 12 and the coupling element 18 arecapable of defining annular free space around the entire perimeter ofthe optical fiber element 12, annular free space will likely be presentwithin the optical fiber cable 10, thereby reducing unwantedmicrobending and macrobending effects.

The coupling element 18 according to the present invention is made of acoupling material that is substantially incompressible but that willreadily deform under light loading. An exemplary coupling element 18 isa coupling tape, though the coupling element 18 can be formed in-situ,such as via melt extrusion.

By way of example, this coupling material possesses excellentelasticity; at standard temperature and pressure, it has an elongationto break of at least about 100 percent, typically at least about 250percent, and more typically at least about 500 percent. In someinstances, the coupling composition may possess an elongation to breakof at least about 1,000 percent or more (e.g., about 1,500 percent).Furthermore, this deformable but substantially incompressible materialis tear resistant and durable; at standard temperature and pressure ithas tensile strength of at least about 100 psi and typically at leastabout 400 psi. In some instances, the coupling material may possess atensile strength of about 1,000 psi or more.

As used herein, standard temperature and pressure refers to testingconditions of 50 percent relative humidity at 70° F. (i.e., about 20°C.) and atmospheric pressure (i.e., 760 torr).

Furthermore, the coupling element 18 itself is generally structured todeform rather than to compress when subjected to applied force. Forinstance, the coupling element 18 is typically not a compressible foamadjacent to the optical fiber element 12 (i.e., the innermost portion ofthe coupling element 18 where dynamic coupling of the optical fiberelement 12 occurs).

Those having ordinary skill in the art will appreciate that, as ageneral matter, solids are incompressible. Many seemingly solidstructures (i.e., colloidal foams) will compress under pressure. Thisapparent compression, however, is thought to be the result of eithercompression (e.g., closed-cell foams) or ejection (e.g., open-cellfoams) of entrained gas bubbles rather than compression of the solidmaterial itself.

The coupling element 18 of the present invention embraces materialsthat, though not compressible, will deform upon interference contactwith another material object. Such deformable yet substantiallyincompressible materials are typically solids, but embrace non-solids,too, such as viscous liquids and cohesive gels (e.g., an oil-expandedpolymeric material).

As used herein, “interference contact” is meant to describe physicalcontact between two objects in which one of the objects is compressed,deformed, or otherwise displaced by the other (i.e., more than meretouching).

Exemplary materials for the coupling element 18 include low-porositysolid gels, soft rubbers, soft cross-linkable silicones, and variouspolymers (e.g., styrene-butadiene, styrene-isoprene-styrene,styrene-ethylene/butylene-styrene, andstyrene-ethylene/propylene-styrene) plasticized with mineral oils or,more typically, synthetic oils.

In this regard, plasticizers, which have an affinity for polymers, canbe included in amounts between 10 and 95 weight percent. As forthherein, the present invention typically employs plasticizers at theupper end of this range (e.g., more than 50 weight percent oil).

In addition, the materials employed for the coupling element 18 canfurther include lubricants, which have an aversion to the polymers andplasticizers, in amounts up to about 5 weight percent. As will be knownto those having ordinary skill in the art, surface migration oflubricants reduces friction between the coupling material and processingequipment, thereby facilitating polymer processing (e.g., extrusion). Ingeneral, the coupling materials for use in the present optical fibercable 10 may include a lubricant or similar processing aid.

It has been discovered that blending super-high molecular weightpolymeric materials and oils that are capable of flowing at lowtemperatures (i.e., possessing a low pour point) can yield soft,cohesive gels (i.e., coupling materials) that provide exceptionalperformance. The cohesive gels formed from these polymer/oil blendsusually possess a melt flow temperature of at least about 80° C. (e.g.,90° C. or more). Typical polymer/oil blend ratios are between 30:70 and5:95, more typically less than 15:85 (e.g., 8-12 weight percentpolymer), and in some instance less than 10:90 (e.g., 5-9 weight percentpolymer). The tensile strength of the cohesive gels (e.g., plasticizedhigh-molecular weight elastomeric polymers) tends to be proportional tothe weight fraction of the super-high molecular weight polymers.Elasticity (e.g., elongation to break) is roughly comparable acrossthese aforementioned blend ranges (i.e., between about 5 and 30 weightpercent super-high molecular weight polymer).

As used herein and unless otherwise specified, molecular weight refersto number-average molecular weight, rather than weight-average molecularweight.

Excellent elastomeric block copolymers for use in the presentpolymeric/oil blends include styrene-ethylene/butylene-styrenecopolymers (i.e., with an S-EB-S block structure) having anumber-average molecular weight of about 100,000 g/mol or more, andtypically about 200,000 g/mol or more (as measured by gel permeationchromatography) (e.g., 200,000 to 2,000,000 g/mol). The SEBS copolymerspossess excellent weatherability and, at super-high molecular weights,demonstrate elevated elongation to break. An exemplary, super-highmolecular weight styrene-ethylene/butylene-styrene block copolymer iscommercially available under the trade name KRATON G-1651.

As will be appreciated by those having ordinary skill in the art,super-high molecular weight elastomeric block copolymers, particularlystyrene-ethylene/butylene-styrene copolymers, have not been successfullyemployed commercially in communication cables. Plasticizing even modestweight fractions (e.g., 3 weight percent or more) of super-highmolecular weight styrene-ethylene/butylene-styrene block copolymers isexceedingly difficult, requiring unconventionally high processingtemperatures (e.g., 250-350° F.). In addition, the relative highfractions of oil (e.g., more than about 70 weight percent) depress meltstrength, thereby making extrusion blending and pelletizing impractical.

Further, with respect to the foregoing polymeric/oil blends, outstandingoils do not crystallize or form wax precipitates at low temperatures.Such oils typically possess a pour-point of −25° C. or less (e.g., −30to −35° C.). Moreover, satisfactory oils used in the coupling materialshould plasticize the elastomeric block copolymers but not adverselyaffect the polymeric materials that are conventionally employed forbuffer tube casings, particularly polyolefins.

Accordingly, the most acceptable oils possess low absorbability inpolyethylene or polypropylene, either of which is a cost-effectivepolymeric material suitable for buffer tube casings. If absorbability isrelatively high, conventional polyolefin buffer tubes (e.g.,polyethylene or polypropylene casings) will tend to absorb excessivequantities of oil. As will be appreciated by those having ordinary skillin the art, excessive oil absorption will cause polyolefin buffer tubesto swell and, more importantly, to soften, thereby causing a loss incrush resistance.

Synthetic hydrocarbon oils, such as highly branched isoparaffinicpolyalphaolefins (PAOs), perform well in this regard. Exemplarysynthetic hydrocarbon oils possess a pour-point of −25° C. or less(e.g., −30° C. to −50° C.) and some −40° C. or less (e.g., −45° C. to−65° C.)

In addition, exemplary synthetic hydrocarbon oils possess anabsorbability in polyethylene and/or polypropylene of less than 20percent, typically less than 15 percent (e.g., 8-12 percent), moretypically less than 10 percent (e.g., 4-8 percent), and most typicallyless than 5 percent (e.g., 2-3 percent). As used herein, the concept ofabsorbability describes an oil's ability at 85° C. to saturate aparticular polymer and is measured by that polymer's weight percentincrease as a result of oil absorption to saturation. For instance, anoil having 7 percent absorbability in polypropylene means that a 100gram sample of polypropylene absorbs 7 grams of oil at about 85° C.

The synthetic hydrocarbon oils (e.g., PAOs) typically possess aviscosity of 2-40 centistokes at 100° C., more typically 5-9 centistokesat 100° C. (e.g., 6-8 centistokes at 100° C.). An exemplary syntheticoil is available from Chevron Phillips under the trade name SYNFLUIDPAO.

In accordance with the foregoing, one exemplary cohesive gel for use asa coupling material is formed from a blend of super-high molecularweight styrene-ethylene/butylene-styrene (SEBS) copolymers (e.g.,possessing a number-average molecular weight of about 200,000 g/mol orhigher) and synthetic hydrocarbon oils, particularly polyalphaolefins(PAOs), having a pour-point of less than about −25° C. (e.g., −35° C. orless) and an absorbability in polyethylene and/or polypropylene of lessthan about 10 percent (e.g., 1-4 percent). For instance, a PAO oilhaving a viscosity of about 6 centistokes at 100° C. possesses apour-point of −63° C.

Another cohesive gel is formed from a blend of the high molecular weightstyrene-ethylene/butylene-styrene (SEBS) copolymers and synthetichydrocarbon oils having (i) a pour-point of less than about −15° C. andan absorbability in polyethylene and/or polypropylene of less than about5 percent (e.g., 2-3 percent) and/or (ii) a pour-point of less than −35°C. and an absorbability in polyethylene and/or polypropylene of lessthan about 15 percent (e.g., 2-3 percent).

In accordance with the foregoing, these exemplary embodiments of thepolymer/oil cohesive gel can be formed by blending (i) between about 5and 15 weight percent (e.g., 7-13 weight percent) of the super-highmolecular weight styrene-ethylene/butylene-styrene block copolymers,such as those available under the trade name KRATON G-1651 and (ii) atleast about 85 weight percent of a polyalphaolefinic synthetic oilhaving a viscosity of 5-8 centistokes at 100° C., such as that availableunder the trade name SYNFLUID PAO.

One or more of the foregoing coupling materials are disclosed in U.S.Provisional Patent Application Ser. No. 60/946,754, for CouplingComposition for Optical Fiber Cables (filed Jun. 28, 2007) and U.S.patent application Ser. No. 12/146,588, for Coupling Composition forOptical Fiber Cables, filed Jun. 26, 2008 (and published Jan. 1, 2009,as Publication No. 2009/0003785 A1), each of which is herebyincorporated by reference in its entirety.

In accordance with the foregoing, the thickness of the coupling element18 is sized relative to the optical fiber element 12 so as to create agap within the buffer tube 16 (e.g., between the optical fiber element12 and the other elements that make up the optical fiber cable 10).Those having ordinary skill in the art will recognize that a gap aroundthe entire perimeter of the optical fiber element 12 constitutes annularfree space.

With reference to FIG. 3, the sum of the thicknesses of the elementspositioned within the buffer tube 16 (i.e., the optical fiber element12, the water-swellable element 14, and the coupling element 18) is lessthan the inside diameter of the buffer tube 16. This gap within thebuffer tube 16 helps to reduce undesirable (and perhaps unnecessary)contact points, thereby controlling unnecessary contact pressures frombeing exerted on the optical fiber element 12. Accordingly, the couplingelement 18 does not compress the optical fiber element 12 under mostoperational circumstances.

Additionally, the gap allows the optical fiber element 12 to moveaxially on a small, localized scale with respect to the buffer tube 16.This helps prevent the kinds of bunching of the optical fiber element 12that can lead to microbending and macrobending. At the same time,though, the coupling provided by the coupling element 18 preventssubstantial lengthwise movement of the optical fiber element 12 withinthe buffer tube 16.

FIG. 4 depicts a partial sectional view of the inner wall of the opticalfiber cable 10 according to the present invention along line 2-2 ofFIG. 1. A buffer tube 16 surrounds a water-swellable element 14, whichin turn surrounds a deformable yet substantially incompressible couplingelement 18. These layers enclose an optical fiber element 12 to form theoptical fiber cable 10.

In particular, FIG. 4 illustrates deformation of the coupling element 18under forces applied by the optical fiber element 12 (e.g., interferencecontact). The material displaced by the imposition of the optical fiberelement 12 into the deformable profile of the coupling element 18temporarily forms a protruding shelf-like structure 20, which canrestrict axial movement of the optical fiber element 12 within thebuffer tube 16. In this way, the coupling element 18 serves todynamically couple the optical fiber element 12 to the buffer tube 16.

Those having ordinary skill in the art will appreciate that the opticalfiber cable 10 of the present invention can be viewed as including acentral cable core positioned within a buffer tube 16. The core itselfcontains the optical fiber element 12, which is at least partlypositioned within the coupling element 18. In accordance with thepresent invention, at one or more cross-sections of the optical fibercable 10, the coupling element 18 defines a minimum coupling diameterthat is greater than the maximum cross-sectional width of the opticalfiber element 12. See FIG. 3.

The core may further include a water-swellable element 14, which ispositioned adjacent to the coupling element 18, typically opposite theoptical fiber element 12. See FIG. 3. The water-swellable element 14helps to block the ingress of water into the core or, if water intrusionoccurs, to impede the flow of water along the length of the opticalfiber cable 10.

As noted, the structure and composition of the coupling element 18allows it to deform upon interference contact with the optical fiberelement 12 to create a shelf-like protrusion 20 (e.g., a bulge) thatopposes axial movement of the optical fiber element 12 with respect tothe buffer tube 16.

FIGS. 6 and 7 depict cross-sectional views of alternative embodiments ofthe present invention. In these embodiments, the coupling element 18includes multiple strips or beads of coupling material positionedintermittently around the optical fiber element 12. These alternativeembodiments are intended to illustrate the variety of configurationsembraced by the present invention.

In the exemplary cable embodiment depicted in FIG. 6, an optical fiberelement 12 is disposed within a buffer tube 16. A water-swellableelement 14 is positioned adjacent to the inner wall of the buffer tube16 between the optical fiber element 12 and the inner wall of the buffertube 16. Three strips of a deformable yet substantially incompressiblematerial, working together as a coupling element 18, are intermittentlypositioned adjacent to the optical fiber element 12 between the opticalfiber element 12 and the water-swellable element 14.

In another exemplary cable embodiment depicted in FIG. 7, an opticalfiber element 12 is disposed within a buffer tube 16. Alternating strips(or beads) of a coupling element 18 and a water-swellable element 14 arepositioned adjacent to the inner wall of the buffer tube 16 between theoptical fiber element 12 and the inner wall of the buffer tube 16.Typically, the coupling element 18 itself is deformable yetsubstantially incompressible.

In one particular embodiment, the invention embraces an optical fibercable 10 containing an optical fiber element 12 enclosed with a buffertube 16. The optical fiber element 12 includes at least one opticalfiber (e.g., a single optical fiber, at least two optical fibers twistedtogether, or a ribbon stack). A coupling element 18 made up of adeformable yet substantially incompressible material is positionedbetween the optical fiber element 12 and the buffer tube 16. Typically,the coupling element 18 itself is substantially incompressible.

The coupling element 18 serves to couple the optical fiber element 12 tothe buffer tube 16. The optical fiber cable 10 may further include awater-swellable element 14 (e.g., a water-swellable tape) disposedbetween the buffer tube 16 and the coupling element 18.

FIGS. 8 and 9 depict cross-sectional views of further alternativeembodiments of the present invention.

In these embodiments, the coupling element 18 is a strip (or bead)formed from the deformable yet substantially incompressible couplingmaterial. The coupling element 18 is positioned between the opticalfiber element 12 and the inner wall of the buffer tube 16.

As depicted in FIG. 8, the coupling element 18 and the water-swellableelement 14 are positioned adjacent to the inner wall of the buffer tube16 between the optical fiber element 12 and the inner wall of the buffertube 16. (The optical fiber cable embodiment depicted in FIG. 9, ofcourse, omits a water-swellable element 14.)

In either of these particular optical fiber cable embodiments, thecoupling element 18 may be a continuous or an intermittent bead of thecoupling composition positioned along the length of the optical fiberelement 12 (e.g., configured in a straight or helical strip). Thosehaving ordinary skill in the art will appreciate that, in suchsingle-bead configurations, the coupling element 18 provides somewhatless protection to the optical fiber element 12, but provides adequatecoupling of the optical fiber element 12 to the buffer tube 16.

Additionally, reinforcing rods may be included to provide supplementalstiffness to the buffer tube 16, thereby inhibiting bending. Thereinforcing rods, for instance, may be incorporated within the structureof the buffer tube 16. As will be known by those having ordinary skillin the art, reinforcing rods may be formed from glass-reinforcedplastic. In this regard, exemplary glass-reinforced plastic (GRP) mightinclude between about 80 and 90 weight percent glass.

As noted previously, the coupling element 18 should be configured andsized to retain a gap between the coupling element 18 and the opticalfiber element 12—at least for some discrete segments along the opticalfiber cable 10. In other words, the optical fiber element 12 should becapable of being positioned within the buffer tube 16 in a way that doesnot require contact with the coupling element 18 (i.e., capable ofdefining annular free space).

At any given cable cross-section that includes the coupling element 18,the coupling element 18 provides coupling area for the optical fiberelement 12. See FIG. 2 (depicting a coupling space cross-section). Inthis regard, the concept of “nominal coupling diameter” is used hereinto characterize free area available to the optical fiber element 12within the inner region of the optical fiber cable 10 (e.g., within thecoupling element 18 and the buffer tube 16).

In particular, as used herein, the term “nominal coupling diameter”describes, at any given cable cross-section that includes the couplingelement 18, the diameter of the largest circular cross-section (i.e.,circle) that fits within the inner area defined by the coupling element18 (and the buffer tube 16 or other surrounding cable components)without touching the coupling element 18. See FIG. 10. Furthermore andas used herein, a cable cross-section that does not include (i.e.,intersect or otherwise traverse) the coupling element 18 does not definea nominal coupling diameter.

Likewise, the optical fiber element 12 defines a maximum cross-sectionalwidth. As used herein, the term “maximum cross-sectional width” is meantto characterize the largest cross-sectional dimension of the opticalfiber element 12. For example, for an optical fiber bundle having acircular cross-section, the maximum cross-sectional width is simply thediameter of the circle it defines. For a rectangular ribbon stack, themaximum cross-sectional width is the measurement of its diagonal.

In accordance with one embodiment of the optical fiber cable 10, at oneor more cross-sections of the optical fiber cable 10, the nominalcoupling diameter of the coupling element 18 is sized so that it exceedsthe maximum cross-sectional width of the optical fiber element 12. Inother words, along the length of the optical fiber cable 10 there is atleast one cross-section in which the coupling element 18 is sized suchthat the optical fiber element 12 can be positioned (e.g., configured)within the buffer tube 16 (and adjacent to the coupling element 18)without contacting the coupling element 18.

In accordance with another embodiment of the optical fiber cable 10, atany cross-section of the optical fiber cable 10, the nominal couplingdiameter of the coupling element 18 is sized so that it exceeds themaximum cross-sectional width of the optical fiber element 12. Statedotherwise, this embodiment describes an optical fiber cable 10 in whichthe optical fiber element 12 is capable of defining annular free spacewithin the entire length of the optical fiber cable 10 (i.e., capable ofcontact-free placement). Those having ordinary skill in the art willappreciate that in this embodiment the optical fiber element 12 is neversqueezed or otherwise subjected to compressive forces within thecoupling element 18 at any point along the length of the optical fibercable 10. In other words, the coupling element 18 protrudes into thebuffer tube 16 toward the optical fiber element 12, but does not createforced contact with the optical fiber element 12. See e.g., FIGS. 3 and10.

More generally, in either of the foregoing optical fiber cableembodiments, the coupling element 18 does not completely fill the regionbetween the inside wall of the buffer tube 16 and the optical fiberelement 12.

In yet another particular embodiment, the buffer tube 16 and couplingelement 18 are sized such that, along a segment of the optical fibercable 10 (e.g., over a meaningful length), the coupling element 18defines a continuum of nominal coupling diameters, the average of whichis greater than the maximum cross-sectional width of the optical fiberelement 12.

The buffer tube 16 and coupling element 18 may further be sized suchthat, along a segment of the optical fiber cable 10, the minimum nominalcoupling diameter of the coupling element 18 is greater than the maximumcross-sectional width of the optical fiber element 12. In other words,the maximum cross-sectional width of the optical fiber element 12 doesnot exceed or equal the clearance provided by (i.e., within) thecoupling element 18 for that segment of the optical fiber cable 10. Inthis way, the coupling element 18 never causes any squeeze coupling orsqueeze contact of the optical fiber element 12 along the particularsegment of the optical fiber cable 10.

In either configuration, interference contact of the optical fiberelement 12 with the coupling element 18 creates a shelf-like protrusion20. This shelf-like protrusion 20 resists axial movement of the opticalfiber element 12 with respect to the buffer tube 16.

The components of the optical fiber cable 10 should be designed toensure that the optical fiber element 12 is not squeezed or otherwisesubjected to compressive forces within the coupling element 18. Asillustrated in FIG. 11, for a buffer tube 16 having a specified innerdiameter, the coupling element 18 should be thin enough such that theinner diameter of the buffer tube 16 is greater than the combinedthicknesses of all elements within the buffer tube 16. For example, thesum of twice the thickness of the coupling element 18 (i.e., t₁₈), twicethe thickness of any other layers within the buffer tube 16 (e.g., awater-swellable element 14), and the maximum cross-sectional width ofoptical fiber element 12 (i.e., t₁₂) should be less than the innerdiameter of the buffer tube 16 (i.e., t₁₆). See FIG. 11. (Those havingordinary skill in the art will recognize that the thicknesses of thecoupling element 18 and other layers within the buffer tube 16 areaccounted for twice because these elements encircle the optical fiberelement 12.)

By way of example and in view of the foregoing, a rectangular ribbonstack according to the present invention may be formed with or without acentral twist (herein referred to as a “primary twist”). Those havingordinary skill in the art will appreciate that a ribbon stack istypically manufactured with rotational twist to ensure that itsconstituent optical fibers travel the same distance during cablewinding. In either instance (i.e., with or without twist), the maximumcross-sectional width of the ribbon stack is simply its diagonal length.See FIG. 10.

In a structural variation, a twisted (or untwisted) rectangular ribbonstack may be further formed into a coil-like or a wave-likeconfiguration (i.e., having regular “secondary” deformations). For anyoptical fiber element 12, these secondary configurations define an“effective maximum width.”

For example, a rectangular ribbon stack may be formed into a helicalconfiguration (i.e., a helix) in which the outermost diameter defined bythe helix (i.e., its “effective maximum width”) exceeds the maximumcross-sectional width of the rectangular ribbon stack. Similarly, arectangular ribbon stack may be formed into a wave-like configuration(e.g., sinusoidal) in which the peak-to-trough measurement (i.e., its“effective maximum width” is twice its amplitude) exceeds the maximumcross-sectional width of the rectangular ribbon stack.

Accordingly, in another embodiment of the present invention, the opticalfiber element 12 is configured to have regular secondary deformationssuch that its effective maximum width exceeds its maximumcross-sectional width. Moreover, in this embodiment, the nominalcoupling diameter of the coupling element 18 is typically sized so thatit exceeds the effective maximum width of the optical fiber element 12.

By way of further illustration, a twisted rectangular ribbon stack maybe formed into a helical configuration such that (i) the maximumcross-sectional width of the optical fiber element 12 is less than itseffective maximum width and (ii) the effective maximum width of theoptical fiber element 12 is less than the nominal coupling diameter ofthe coupling element 18. Stated otherwise, the optical fiber element 12(having primary twists and configured into a helix) can be positionedwithin the buffer tube 16 (and adjacent to the coupling element 18) suchthat the twisted and helical optical fiber element 12 is not in contactwith the coupling element 18.

In accordance with the foregoing, in this alternative structure, thecoupling element 18 provides space to account for any regular eccentricmovement (e.g., helical coiling) of the optical fiber element 12 withinthe buffer tube 16. That is, the coupling space within the buffer tube16 (and the coupling element 18) should accommodate the effectivemaximum width of the optical fiber element 12.

More typically, a twisted (or untwisted) rectangular ribbon stack mayassume irregular “secondary” deformations (e.g., irregular coiling)during use (e.g., installation). Accordingly, the nominal couplingdiameter of the coupling element 18 can be sized so that the couplingelement 18 facilitates coupling of the optical fiber element 12 whendetrimental levels of excess length occur within optical fiber cable 10.The design considerations of the optical fiber cable 10 should balancethe extremes of excess length and interference contact between theoptical fiber element 12 and the coupling element 18. As noted, this isprincipally controlled by ensuring that at one or more cross-sections ofthe optical fiber cable 10, the nominal coupling diameter of thecoupling element 18 is sized so that it exceeds the maximumcross-sectional width of the optical fiber element 12.

Whether its secondary deformations are regular or irregular, the opticalfiber element 12 typically rests on the coupling element 18 at discretelocations along the length of the optical fiber cable 10 (i.e., opticalfiber element 12 most typically does not contact the coupling element 18along the entire length of the optical fiber cable 10). Theseinterference contacts are sufficient to support the optical fiberelement 12 within the buffer tube 16. In this way, the optical fiberelement 12 can move relative to the buffer tube 16 to account for thedifferences in the thermal expansion or contraction of the differentconstituent materials.

By way of example, when the buffer tube 16 contracts relative to theoptical fiber element 12 due to thermal contraction or bending of theoptical fiber cable 10, the difference in length between the buffer tube16 and the optical fiber element 12 is known as “excess length.” Whenexcess length develops in the optical fiber element 12, the opticalfiber element 12 may coil or bend, thereby creating interference contactwith the coupling element 18. Upon such contact, the coupling element 18supports the optical fiber element 12 by deforming to create ashelf-like protrusion 20. In response to the application of such aninterference force, the coupling element 18 resists axial movement ofthe optical fiber element 12 within the buffer tube 16.

For instance and as noted, various conditions may cause excess length inthe optical fiber element 12 to bunch, slump, gather, or otherwisecollect in a small section of the buffer tube 16. For instance, for anoptical fiber cable 10 oriented vertically, gravity tends to causeexcess length in the optical fiber element 12, the excess lengthtypically collecting near the bottom of the optical fiber cable 10.

Likewise, in optical fiber cables 10 that contract after having beenstretched during installation or thermally expanded, the componentoptical fiber element 12 may tend to bunch. As depicted in FIG. 4, thecoupling element 18 resists these tendencies of the optical fiberelement 12 to slump by deforming to create a shelf-like protrusion 20wherever interference contact between the optical fiber element 12 andthe coupling element 18 occurs.

The coupling element 18 may embrace various forms to effectively couplethe optical fiber element 12 to the buffer tube 16.

For instance, in the embodiment depicted in FIG. 1, the coupling element18 may be a deformable yet substantially incompressible material thatcompletely encircles the optical fiber element 12 (i.e., acircumferentially continuous layer). By completely surrounding theoptical fiber element 12, this configuration ensures the buffer tube 16and any components within the buffer tube 16 cannot exert amicrobending-inducing force or macrobending-inducing force on theoptical fiber element 12.

The circumferentially continuous layer may be lengthwise-continuous,too, to ensure that coupling can be achieved at any point along thelength of the optical fiber cable 10. Alternatively, thecircumferentially continuous layer may instead be intermittently spacedalong the length of the optical fiber cable 10. See FIG. 5. To accountfor the often non-linear orientation of the optical fiber element 12within the buffer tube 16, the resulting intermittent rings of couplingmaterial should be spaced closely enough to effectively engage theoptical fiber element 12 regardless of how the optical fiber element 12twists or bends within the buffer tube 16. This configuration reducesthe amount of material used to form the coupling element 18 yetmaintains sufficient coupling of the optical fiber element 12 to thebuffer tube 16.

Furthermore, in the embodiments depicted in FIGS. 6 and 7, the couplingelement 18 may be provided as a plurality of strips or beads of adeformable yet substantially incompressible material spaced around theperimeter of the optical fiber element 12 (i.e., circumferentiallydiscontinuous). The number and spacing of the strips of material shouldbe sufficient to effectively engage the optical fiber element 12regardless of the orientation of the optical fiber element 12. Stateddifferently, the gaps between the intermittent strips should not be sogreat as to readily permit an irregularly coiled optical fiber element12 to become positioned between the strips and, possibly, to contact thewater-swellable element 14 or the buffer tube 16. In this configuration,less material is used to form the coupling element 18, but the couplingelement 18 nonetheless provides adequate coupling of the optical fiberelement 12 to the buffer tube 16.

In this circumferentially discontinuous variation, the strips or beadsof deformable yet substantially incompressible material may be providedas lengthwise-continuous strips that provide uniform coupling at anypoint along the length of the optical fiber cable 10. Alternatively, thestrips or beads may be lengthwise-discontinuous (e.g., segments ofdeformable yet substantially incompressible material), using only somuch material as is necessary to provide the desired coupling and fiberprotection.

In conjunction with these circumferentially discontinuous alternatives,the water-swellable element 14 may be provided as water-swellable strips(e.g., yarns containing water-swellable material). As depicted in FIG.7, the water-swellable strips may be positioned between the lengthwisestrips of the coupling material. This configuration uses lesswater-swellable materials to perform the desired water-blockingfunction. Further, by alternating the water-swellable element 14 andcoupling element 18 about the inner surface of the buffer tube 16 ratherthan stacking the elements on top of each other, thinner buffer tubesmay be constructed while retaining significant coupling andwater-blocking functions.

The coupling element 18 can be formed and/or configured within thebuffer tube 16 in various ways. For example, the coupling element 18 maybe extruded onto the buffer tube 16 (or onto the water-swellable element14, if present) so as to at least partially surround the optical fiberelement 12.

In this regard, in one particular embodiment, the coupling element 18may be co-extruded with the buffer tube 16 to not only surround theoptical fiber element 12 but also bond the coupling element 18 and thebuffer tube 16 together. Further, the coupling element 18,water-swellable element 14, and buffer tube 16 may all be extrudedtogether.

In another particular embodiment, the coupling element 18 may be formedseparately from the buffer tube 16 and subsequently secured to thebuffer tube 16 such as by frictional coupling, by thermal bonding, or byadhesive bonding. For instance, a thermoplastic or thermoset adhesivemay be applied to couple the coupling element 18 and the buffer tube 16.

As discussed previously, the coupling element 18 can be composed ofmaterials that exhibit an elastic reaction to an applied force (e.g.,polymers). In a particular embodiment, the coupling element 18 possesseswater-blocking characteristics, thereby reducing if not eliminating theneed for a separate water-swellable element 14.

For instance, the coupling element 18 might be formed fromwater-swellable materials or a blend thereof (e.g., a blend ofwater-swellable polymers and non-water-swellable polymers). In thisregard, exemplary water-swellable materials include a matrix (e.g.,ethylene vinyl acetate or rubber) enhanced with about 30-70 weightpercent super absorbent polymers (SAPs), such as particulates of sodiumpolyacrylate, polyacrylate salt, or acrylic acid polymer with sodiumsalt. Such water-swellable materials can be processed on conventionalhot melt adhesive machinery. An exemplary water-swellable material,which can be further blended with non-water-swellable polymeric materialto enhance its elasticity, is available from the H. B. Fuller Companyunder the trade name HYDROLOCK.

Alternatively, the coupling element 18 can be enhanced withwater-swellable particulate powders, which can be bound, for instance,to the surface of the coupling element 18, usually opposite the opticalfiber element 12 to reduce the risk of optical attenuation (e.g.,microbending) or glass degradation. Such powders are typically composedof super absorbent polymers (SAPs) that, when bound on or impregnated inthe coupling element 18, are dry to the touch and, accordingly, arereadily removed from cables during splicing operations. Moreover, thewater-swellable particulate powders can be applied to the outer surfaceof the coupling element 18 (i.e., opposite the optical fiber element12), either completely or partially (e.g., intermittently).

As noted, in another aspect, the invention embraces methods for makingthe optical fiber cable 10 as previously described.

The method typically includes forming an enclosed buffer tube 16,positioning an optical fiber element 12 having at least one opticalfiber within the buffer tube 16, and positioning a coupling element 18between the optical fiber element 12 and the buffer tube 16. The methodmay further include positioning a water-swellable element 14 between thecoupling element 18 and the buffer tube 16.

The coupling element 18, which is typically formed from a deformable yetsubstantially incompressible material, is configured to define freespace between the optical fiber element 12 and the coupling element 18.In this regard, the coupling element 18 is usually positioned betweenthe optical fiber element 12 and the buffer tube 16 such that, at across-section of the optical fiber cable 10, the coupling element 18defines a nominal coupling diameter that exceeds the maximumcross-sectional width of the optical fiber element 12.

In accordance with the foregoing, the step of positioning the couplingelement 18 between the optical fiber element 12 and the buffer tube 16may embrace placing a plurality of strips of deformable yetsubstantially incompressible material along the length of the buffertube's internal surface 16. These strips of coupling material may becontinuous or discontinuous (i.e., dash-like segments) along the lengthof the buffer tube 16 and further, may be intermittently spaced aboutthe perimeter of the optical fiber element 12.

The steps of forming an enclosed buffer tube 16 and positioning anoptical fiber element 12 within the buffer tube 16 may embrace, forinstance, extruding the buffer tube 16 around the optical fiber element12. Those having ordinary skill in the art will appreciate that thebuffer tube 16 can be formed from various polymeric materials, such aspolyethylene or polypropylene.

The step of positioning a coupling element 18 between the optical fiberelement 12 and the buffer tube 16 may be achieved, for instance, byco-extruding the coupling element 18 and buffer tube 16 around theoptical fiber element 12.

Further still, the steps of forming the enclosed buffer tube 16,positioning the coupling element 18 between the optical fiber element 12and the buffer tube 16, and positioning a water-swellable element 14between the coupling element 18 and the buffer tube 16 may involveco-extruding the outer buffer tube 16, the water-swellable element 14,and the coupling element 18 around the optical fiber element 12.

An alternate method of constructing an optical fiber cable 10 includesthe step of forming a core from a substantially incompressible couplingelement 18, which is placed around an optical fiber element 12 having atleast one optical fiber.

In forming the core, a water-swellable element 14 may further bepositioned around the coupling element 18. The core is then enclosedwithin a buffer tube 16 to form the optical fiber cable 10. The methodmay further include the step of securing the coupling element 18 to thebuffer tube 16 (e.g., by applying an adhesive, by melt-bonding, or byfrictional coupling).

The step of forming the core may be completed in various ways. Forinstance, deformable yet substantially incompressible material may beextruded around the optical fiber element 12. In another way, thecoupling element 18 may be a tape of deformable yet substantiallyincompressible material that is bent, wrapped, or otherwise shaped toform a tube around the optical fiber element 12 (e.g., the sides of thetape may be bonded to form a convolute structure about the optical fiberelement 12).

In yet another way, the steps of forming the core and enclosing the corewithin a buffer tube 16 may be performed together. In this regard, thecoupling element 18 may be a plurality of lengthwise strips ofdeformable yet substantially incompressible material positioned aboutthe cross-sectional perimeter of the optical fiber element 12 andsecured to the buffer tube 16.

Regardless of how the core is formed, though, for at least onecross-section of the core, the coupling element 18 should define anominal coupling diameter that exceeds the maximum width of the opticalfiber element 12. This ensures that a gap is present with the internalregion of the optical fiber cable 10.

In the specification and figures, typical embodiments of the inventionhave been disclosed. The present invention is not limited to suchexemplary embodiments. Unless otherwise noted, specific terms have beenused in a generic and descriptive sense and not for purposes oflimitation.

1. An optical fiber cable, comprising: an optical fiber elementincluding an optical fiber ribbon stack; a buffer tube enclosing saidoptical fiber element; and a deformable yet substantially incompressiblecoupling element positioned between said optical fiber element and saidbuffer tube; wherein, at a cross-section of said optical fiber cable,said coupling element defines a nominal coupling diameter; and whereinthe nominal coupling diameter of said coupling element is greater thanthe maximum cross-sectional width of said optical fiber element.
 2. Anoptical fiber cable according to claim 1, comprising a water-swellableelement positioned between said coupling element and said buffer tube.3. An optical fiber cable according to claim 1, wherein said opticalfiber element is capable of being positioned within said buffer tube ina way that does not require contact with said coupling element.
 4. Anoptical fiber cable according to claim 1, wherein said optical fiberelement manifests secondary deformations, said secondary deformationsdefining an effective maximum width for said optical fiber element thatexceeds the maximum cross-sectional width of said optical fiber element5. An optical fiber cable according to claim 1, wherein said couplingelement possesses an elongation to break of at least about 100 percentand a tensile strength of at least about 100 psi at standard pressureand temperature.
 6. An optical fiber cable according to claim 1, whereinsaid coupling element possesses an elongation to break of at least about250 percent and a tensile strength of at least about 400 psi at standardpressure and temperature.
 7. An optical fiber cable according to claim1, wherein said coupling element possesses an elongation to break of atleast about 500 percent and a tensile strength of at least about 1,000psi at standard pressure and temperature.
 8. An optical fiber cableaccording to claim 1, wherein said coupling element comprises a blend of(i) elastomeric block copolymers having an number-average molecularweight of at least about 100,000 g/mol and (ii) hydrocarbon oilpossessing a pour-point of −15° C. or less and absorbability inpolyethylene or polypropylene of less than 20 percent, said blendpossessing a melt flow temperature of at least about 80° C.
 9. Anoptical fiber cable according to claim 1, wherein said coupling elementcomprises a blend of (i) elastomeric block copolymers having annumber-average molecular weight of at least about 200,000 g/mol and (ii)hydrocarbon oil possessing a pour-point of −25° C. or less andabsorbability in polyethylene or polypropylene of less than 20 percent,said blend possessing a melt flow temperature of at least about 80° C.10. An optical fiber cable according to claim 1, wherein said couplingelement comprises a blend of (i) SEBS block copolymers having annumber-average molecular weight of at least about 200,000 g/mol and (ii)polyalphaolefin (PAO) oil possessing a pour-point of less than about−30° C. and absorbability in polyethylene or polypropylene of less thanabout 15 percent.
 11. An optical fiber cable according to claim 1,wherein said coupling element comprises water-swellable material.
 12. Anoptical fiber cable according to claim 1, wherein, at any cablecross-section that includes said coupling element, the nominal couplingdiameter of said coupling element is greater than the maximumcross-sectional width of said optical fiber element.
 13. An opticalfiber cable according to claim 1, wherein, upon interference contactbetween said optical fiber element and said coupling element, saidcoupling element deforms to create a bulge that resists axial movementof said optical fiber element with respect to said buffer tube.
 14. Anoptical fiber cable according to claim 1, wherein, at any cablecross-section that includes said coupling element, the nominal couplingdiameter of said coupling element exceeds the effective maximum width ofsaid optical fiber element.
 15. An optical fiber cable according toclaim 1, wherein: along a segment of the optical fiber cable, saidcoupling element defines a continuum of nominal coupling diameters; andthe average nominal coupling diameter of said segment of the opticalfiber cable is greater than the maximum cross sectional width of saidoptical fiber element.
 16. An optical fiber cable according to claim 1,wherein said coupling element substantially encircles said optical fiberelement.
 17. An optical fiber cable according to claim 1, wherein saidcoupling element is continuous along the length of said buffer tube. 18.An optical fiber cable according to claim 1, wherein said couplingelement comprises one or more discontinuous beads of deformable yetsubstantially incompressible material.